Aaha! There's the culprit!

Wax eye/White eye/Silver eye/Tauhou

I checked out the crabapple tree across the road from work to see if there were any waxeyes feasting on the ripening fruit. There were but there are still too many leaves on the tree to really get a good shot. As I leaned around the tree to try for a better angle I noticed the activity in the grape vine.

There were about a dozen of the wee beasties having a blast piercing all the grapes. I think they were doing a 'bags this one, and this one, and this one too' without really stopping to eat any of them. The trouble is that you could see the movement but they are so well camoflaged that they were hard to spot and focus on.